This is a problem that was pointed out by a user on the forums: https://forum.unity.com/threads/videoplayer-materialoverride-how-to-set-aspect-ratio-scale-or-change-tiling-offest.772220/

How to reproduce:

1. Open attached project: MaterialOverrideLockedControls
2. Open TestScene
3. Click on the Video Plane game object.
4. Go in the game object's inspector for the Video Material.
5. Observe that you can edit the Main Maps' Tiling and Offset values, both X and Y.
6. Enter playmode
7. Observe that the Tiling and Offset values get reset to 1 and 0, respectively.
8. Try to edit the Tiling and Offset values
9. Observe that they are locked: they should not.

Alternative test:

...
6. Disable the VideoPlayer before entering playmode
7. Enter playmode
8. Try to edit the Main Maps' Tiling and Offset values
9. Observe that they are not locked: as it should.

This problem is present since the VideoPlayer's very first version.